According to a Japanese developer interview, there were plans for each expansion to have their own individual Offline version disc. However in the end, Ambitions of the Illuminus sold quite poorly in Japan. This, on top of a series of ongoing technical problems the Japanese version was facing (PSU sold quite well in Japan, but that didn't translate to paying subscribers) lead to Sega cancelling the third expansion disc in favour of working on PSU's successor (which would go on to become PSO2) a year after Ambitions of the Illuminus had just released.
Man, you always are the best source for this kind of information, my hats off to you. That said, I still wish they patched an offline version of Episode 3 in to the games before shutting down their service to them. Episode 2 leaving on a massive cliff hanger, and then suddenly jumping to PSP2 Infinity where everything is honky-dory has gotta be jarring for people that never got a chance to experience Episode 3, and are playing through the games for the first time.